Implantable cardiac stimulating devices which provide therapeutic electrical stimulation in response to a variety of pathological cardiac arrhythmias are well known in the art. These devices may provide a single type of therapy (e.g., bradycardia pacing therapy) or they may be capable of providing "tiered therapy," in which the type of electrical stimulation administered is determined in accordance with the severity of the arrhythmia, with more aggressive therapies being applied in response to more severe arrhythmias.
Effective delivery of therapy from an implantable cardiac stimulating device depends upon accurate measurement of intrinsic cardiac activity. Devices that provide tiered therapy must not only be capable of detecting the onset of an arrhythmia, but must also be capable of discerning particular types of arrhythmias in order to deliver an appropriate form of electrical stimulation therapy. For example, if a hemodynamically unstable ventricular tachycardia is incorrectly diagnosed as a relatively less severe arrhythmia, valuable time may be lost if an inappropriate, less aggressive therapy, such as antitachycardia pacing, is applied. On the other hand, if a high rate, hemodynamically stable tachycardia is incorrectly diagnosed as ventricular fibrillation, the patient may consciously experience a painful high energy defibrillation shock.
Measurement of intrinsic cardiac activity is also important for devices that provide bradycardia pacing therapy. For instance, a demand pacemaker can inhibit delivery of a pacing pulse when a naturally occurring heartbeat is sensed within a predetermined period of time following a preceding heart beat (the time period commonly referred to as the "escape interval"). Pacing pulse inhibition is desirable because it extends battery life. To achieve this desirable result, the device must be capable of monitoring intrinsic cardiac activity.
Many implantable cardiac stimulating devices detect cardiac arrhythmias by monitoring cardiac electrical activity--i.e., the intracardiac electrogram (IEGM). The IEGM is typically sensed by electrodes that are also used to deliver electrical stimulation to the cardiac tissue. However, under many circumstances, it is difficult to sense the IEGM. For example, the device may not be able to discern the IEGM over external interference. As a result, an implantable cardiac stimulating device may have difficulty detecting the onset of an arrhythmia. As another illustration, implantable cardiac stimulating devices that provide bradycardia pacing therapy may be inhibited from sensing cardiac electrical activity during a period of time immediately following the delivery of a pacing pulse, due to the presence of a pulse-induced after-potential in the vicinity of the pacing electrodes.
Unfortunately, IEGMs are only the electrical trigger signal that proceeds the mechanical activity of the cardiac muscle. While, IEGMs are useful in most cases in determining the rate of contraction of the cardiac muscle, they do not carry information regarding the mechanical vigor of the resulting contraction. To properly assess the performance of the heart as a functioning pump, two pieces of information are needed: the rate of pumping and the amount of fluid displaced. Only rate information is available when a cardiac device monitors the IEGMs. What is needed then, is a sensor which can monitor both rate and the amount of fluid displaced.
Some implantable cardiac stimulating devices monitor physiologic parameters, other than the IEGM, which reflect hemodynamic performance. For example, U.S. Pat. No. 4,774,950 to Cohen refers to a system that detects cardiac arrhythmias by measuring mean pressure at a variety of locations (e.g., mean arterial pressure, mean right ventricular pressure, mean left atrial pressure, mean left ventricular pressure or mean central venous pressure). For a selected mean pressure, a short term current mean pressure is compared to a long term mean baseline pressure, and if they differ by a predetermined value, the patient may be deemed to be experiencing a cardiac arrhythmia. The mean pressure data may also be used in combination with heart rate measurements to detect arrhythmias.
The use of another hemodynamic indicator, blood oxygen level, is described in U.S. Pat. No. 4,967,748 to Cohen. Blood oxygen level is measured at a particular site in the circulatory system of a patient. A comparison is made between a short term sensed blood oxygen level and a baseline blood oxygen level, and if they differ, the patient may be deemed to be experiencing a cardiac arrhythmia.
Unfortunately, the use of hemodynamic indicators such as mean pressure and blood oxygen level may have certain associated drawbacks. One drawback is that certain hemodynamic indicators may not respond rapidly to the onset of an arrhythmia. Thus, an implantable cardiac stimulating device that relies on certain hemodynamic signals to detect cardiac arrhythmias may not deliver therapy as rapidly as desired. Another drawback is that the measurement of certain hemodynamic indicators requires the use of sensors that must be delivered to sites that do not normally receive electrical stimulation. Thus, additional leads may be required, which undesirably add cost to the implantable system and complexity to the surgical procedure during which the leads are implanted.
One proposed solution which overcomes some of the drawbacks associated with the use of the IEGM and certain hemodynamic indicators is described in commonly assigned, copending U.S. patent application Ser. No. 08/091,636, filed 7/14/93, of Causey and Moberg, entitled "Implantable Leads Incorporating Cardiac Wall Motion Sensors and Method of Fabrication and a System and Method for Detecting Cardiac Arrhythmias Using a Cardiac Wall Motion Sensor Signal," which is hereby incorporated by reference in its entirety. That patent application describes implantable leads which incorporate cardiac wall motion sensors that provide signals indicative of cardiac mechanical activity. More particularly, the signals are representative of cardiac wall accelerations as experienced by the motion sensors.
The cardiac wall motion sensors of the above-incorporated patent application rapidly respond to the onset of arrhythmias. The signals are not subject to electrical interference from pacemakers induced after-potentials and external sources. The raw acceleration signals advantageously provide an accurate and reliable indication of cardiac mechanical activity. However, it would be even more desirable to be able to process the raw signals provided by the cardiac wall motion sensors in order to derive signals that strongly correlate to the hemodynamic performance of a patient's heart. It would also be desirable to provide an implantable cardiac stimulating device that uses the derived hemodynamic signals to determine when (and what form of) therapeutic electrical stimulation should be administered.